When a subject is photographed under backlit conditions, the image portion corresponding to the subject is an indistinct backlit image with low lightness and contrast, making it difficult to make out details of the subject. Image processing for enhancing image quality by improving lightness and contrast is performed on such backlit images, as well as images suffering in quality due to overexposure, underexposure, blurring due to jiggling when capturing the image, noise, insufficient lighting, and the like. One method used in conventional image processing is the Retinex process. The Retinex process primarily improves image quality in low-quality regions. In this process, an original image is separated into a reflectance component and an illumination component.
In general, the Retinex process calculates the reflectance components and illumination components from the original pixel values and calibrates the illumination components based on the reflectance components. Next, image calibration is achieved by generating output values using the calibrated illumination components and the original pixel values.
U.S. Pat. No. 6,885,482B1 discloses a method for preventing color imbalance and color slippage that can occur when performing the Retinex process independently on each RGB color plane. In this method, RGB values are converted to a coordinate space, such as YCbCr and YIQ configured of luminance and chrominance components, executing the Retinex process only on the luminance components Y, and restoring the RGB values while keeping the original chrominance components. Hence, this method prevents the occurrence of color imbalance and color slippage by correcting only the luminance component and not the chrominance components. Further, by performing the Retinex process only on the luminance component, fewer calculations are required in this method than in a method performing the Retinex process on each RGB color plane, thereby enabling the process to be performed quickly and eliminating the need to store each of the RGB values for normalizing R(x, y). Since only the luminance signal need be stored, this method reduces the amount of required storage space.
United States Patent Application Publication No. 2004/91164A1 discloses a method for achieving a fast processing speed when performing the Retinas process. In this image processing method, a reduced (low-resolution) image is formed from the original image according to the average pixel method or the like, forming an average peripheral luminance image (blurred image) by finding the average peripheral luminance for each pixel in the reduced image, forming a Retinex process image from the original image and an enlarged image of the blurred image, and forming an output image from this Retinex process image and the original image.